This invention relates to web offset printing presses, and more particularly, it relates to a method and apparatus for determining the dimensional amount of alignment or misalignment of various components of a web offset lithograph printing press, composed of one or more printing units.
Web offset lithograph printing is normally accomplished through use of a three cylinder printing process. The first cylinder is known as a "plate" cylinder, and is surrounded by a metallic image plate. The image is inscribed on the plate and ink is applied to the plate image. The plate cylinder is in adjustable rolling contact with a second cylinder, known as a "blanket" cylinder surrounded by a "blanket" of a rubberized material. The rolling contact of the inked plate on the plate cylinder to the blanket cylinder transfers the inked image from the plate to the blanket. A continuous "web" of paper from a paper roll is driven between the blanket cylinder and a third cylinder known as an "impression cylinder" at a point diametrically opposite the point of contact between plate and blanket cylinders (a point known as the "impression nip"). All cylinders are maintained in impression contact with each other, with the pressure between cylinders known as "squeeze pressure". Squeeze pressure from the cylinders onto the web as the web is driven between the blanket and impression cylinder offsets the inked image from the blanket cylinder to the top of the web.
Most cylinder designs have bearer rings at each end as a major diameter (the largest diameter of the cylinder and bearer ring combination) extending beyond the plate and blanket mounting area. The plate and blanket cylinder body diameter, between the cylinder bearer ends, is undercut to a lesser or "minor" diameter, leaving a space for the plate or blanket. Paper sheets, known as "packing", are wrapped about the cylinder beneath the plate or blanket. Typically, blankets extend four one-thousandths of an inch above the bearer rings, and plates extend one-thousandth of an inch above the bearer rings. During printing the bearer rings are in rolling contact, a condition known as "impression on". The paper sheets, or "underpacking", lift the plate and blanket above the bearer rings and create the printing "squeeze pressure" during "impression on".
The combination of a plate cylinder and a blanket cylinder in rolling contact is known as a "couple". The web can be printed on both sides by using a pair of couples, one with a first plate cylinder rotating above its corresponding blanket cylinder, and one with a second plate cylinder rotating below its corresponding blanket cylinder. The cylinder couple pairs can be arranged with cylinder center lines generally stacked vertically, with cylinder center lines generally set horizontally side-by-side, or with cylinder center lines in a generally staggered vertical stack. The web receiving its inked impression is driven between the two rotating blanket cylinders. When the web is simultaneously printed on both sides in the above manner, each blanket cylinder is the impression cylinder for the other, and the printing unit is known as "perfecting".
Multi-color offset lithograph printing is accomplished through application of different colors of ink in successive stages, with printing accomplished at each stage by individual printing units of the press, each printing unit using the process described above. A single couple with an impression cylinder (or a pair of couples) is used to print the web on one or both sides in a first color; the web is then driven to a subsequent printing unit, also having a single couple with an impression cylinder (or a pair of couples), wherein a second color is printed over the first color image. Full color printing requires four colors (black and the three primary colors) and thus four printing units. Naturally, precise alignment of the various printing units and the components of each printing unit is necessary to ensure that all colors are properly printed and the final mult-color image is sharp and not blurred. Such color-over-color alignment is termed "register".
Efficient operation of a web offset printing press requires proper alignment or register of all components in the press, within one to a few thousandths of an inch. Ideally, all components in a web offset printing press will be exactly aligned. The plate should be properly aligned when wrapped around the plate cylinder so that "register marks" inscribed on the plate coincide with each other, producing "pre-register" of the plate cylinder. The plate cylinder should be exactly aligned with its longitudinal axis parallel to the longitudinal axis of the blanket cylinder if the plate cylinder and plate image are straight. The packing beneath the plate or blanket in both the plate cylinder and the blanket cylinder must properly set the height of the plate or blanket with respect to the bearer rings. In mult-color offset printing, components of each color printing unit must be properly aligned with corresponding components of subsequent and preceding color printing units.
Control of proper alignment or register of the components of an offset web press poses a substantial problem to the printing industry, primarily because misregister can result from a large variety of separate factors. Generally, misalignment can result from three broad areas: a printing press can be improperly designed or manufactured, such that a new press when installed has components out of proper alignment. Alternatively, operation of a press can cause various components of the press to move out of alignment, principally as a result of the vibration or paper jams produced by the press' operation. Finally, various components of the press can be improperly adjusted by the press operators.
Proper alignment or "register" of images, especially in multi-color printing, requires attention to a number of factors. Plate to cylinder register is accomplished by pre-aligning the marks directly opposite each other. Such alignment will also ensure that the plate is square to the plate cylinder, if the image is square to the plate and the plate has been accurately wrapped and locked around the plate cylinder.
Additional misalignment problems can result from the position of the various cylinders with respect to each other. Two kinds of cylinder misalignment are especially troublesome: "angular" error, and "offset" error. Angular error, also known as "misalignment", occurs when cylinders do not have their longitudinal axis or "center lines" parallel. Offset error occurs when the center lines of the cylinders are parallel but the distance between those center lines is improper. Angular and offset error can occur individually, or those errors may occur together in a compound of angular and offset error. Angular and offset error can occur between individual cylinders in a single press unit, or with respect to other press units for different colors. Angular and offset error can also occur with respect to the mounting frames of individual press units.
Other alignment errors are more easily corrected. When the position of a plate cylinder with a plate is laterally improper, the cylinder position can be corrected by moving it along its longitudinal axis toward one side of the press frame. When the position of a plate cylinder with a plate is circumferentially improper, the cylinder position can be corrected by moving it circumferentially around its longitudinal axis. These adjustments are normal register adjustments.
Additionally, the packing surrounding each cylinder must set the plate or blanket to the proper height above the bearer rings. Packing is an intermediate material, usually paper, that surrounds both the plate or blanket cylinders, underneath the plate or blanket, and raises the height of the plate from the cylinder's undercut mounting diameter.
When properly installed, the packed plate and blanket cylinders each apply an opposing predetermined squeeze pressure upon the plate, blanket and paper web. Improper packing can produce improper squeeze pressure at the impression nip. Improper squeeze pressure across the linear impression nip of both the plate and blanket cylinder nips can cause improper ink transfer and/or produce a distorted image.
Each color press unit must also be precisely aligned with preceding and subsequent color press units. Printing presses are normally manufactured with each color press unit in precise three dimensional alignment with the other units. Each press unit is normally affixed to a permanent concrete foundation designed to support the load and avoid drift or shift of the units out of alignment. However, such misalignments can occur during erection or use of the press, or from settling of the foundation, or movement due to shock or other stresses placed upon the press units or their foundations.
Improper register or misalignment of the various components of a web offset printing press results in two principal problems: first, extensive trial and error adjustments, each requiring a printing run, become necessary before the plate images on a multi-color printing process can be placed in placed proper register. Often they cannot register. The plate cylinders often must be cocked, and the lateral and circumferential plate to plate cylinder register often must be varied to ensure proper image register; much paper and time can therefore be wasted. Large printing presses are expensive to operate, so that the preregister maintenance for maximum efficiency will result in expending as little time as possible when making production adjustments on the press, thus leaving the press in a pre-registered and operable condition for a maximum portion of time.
Reduced service life of the press components can also result from misalignment or misregister. The adjustments necessary to place an image in register often require the components of the press to be operated with substantial misalignment (although the resulting image can appear registered). In the case of high-speed, printing presses, such misalignment can cause excessive bearer ring load and squeeze pressure, producing overloads that result in excessive wear of gears, bearings, bearer rings and other rolling contact pressure points, thereby substantially reducing the life of many moving parts in the printing press. It is therefore desirable to discover and correct the source of alignment error to place the printing press printing cylinders and image correctly in register as quickly as possible, and accomplish that register with the various press components having as little misalignment as possible.